Revealing mistake: When Todd is hitting the metal punchbag the first time, during the slow mo part you can see it flex as if it is made of rubber.
Continuity mistake: During the final fight between Cain and Todd, Todd breaks Cain's arm at the elbow and wrist. In the next shot Cain is staggering backwards as Todd winds up for another blow. You can clearly see that Cain's arm is fine and resting comfortably at his side.
Continuity mistake: When the green snake slithers towards the boy, Todd throws a boot at the boys feet so the boy can hit the snake with it. The boy looks at Todd who motions the boy to hit it. The next shot shows the boys feet again, but no boot.
Continuity mistake: When Todd first encounters the humans on the waste disposal planet he falls down some stairs. He coughs up blood which is visible on his hand and on the ground. It cuts away from Todd and when it cuts back there is no blood at all.